A type of interference caused by passive intermodulation products (PIM) occurs in high power radio frequency (RF) communications systems which involve simultaneous transmit and receive operation of RF energy. The problem of PIM generation is particularly acute where the transmit and receive frequencies are closely spaced, which is typically the case for terrestrial and spaced-based communications systems due to the limited availability of frequency spectrum allocated for a specified communication. The loose metal-to-metal contacts which are present within the components of such high power RF communications systems have been identified as sources of PIM.
The generation of intermodulation products in passive metal bodies arises because most metals in air intrinsically possess a thin layer of oxidation which will act as an insulator. When two metal bodies are joined, a metal-insulator-metal interface is produced. Before contact, the insulator acts as a dielectric. As the metal bodies are brought into contact or near contact under pressure, the oxidation layer functions as a semiconductor and sparking will occur. This translates into unwanted signal noise that can be picked up by the receiver resulting in poor system performance.
In the case of a geosynchronous communications satellite, for example, the passive metal-insulator-metal junctions of the transmitter boom assembly have been identified as a source of PIM. It is known in the prior art to use special thermal blankets and/or shields for blocking stray RF energy from contacting the transmitter boom assembly and thereby preventing generation of PIM. These special thermal blankets and/or shields are typically composed of multiple segments or assemblies because the boom assemblies being covered are of irregular geometry and therefore do not lend themselves to an all-in-one encompassing flat pattern wrap or cover.
FIG. 1 illustrates a prior art technique for joining the edges of adjacent RF shielding thermal blankets 10, 12 which cover a metal body 14. In this example, the free edge 11 of the bottom blanket 10 is folded in an S configuration to form a labyrinthine seal with an overlap free edge portion of the top blanket 12. Each blanket 10, 12 is provided with mating hook and loop fasteners 16, 18, respectively, to complete the sealed joint. As is readily apparent, the installation of such an overlap joint can be difficult and time consuming. Also, such joints, especially when located along a bend region, do not always make a perfect seal and therefore present leak paths whereby RF energy may penetrate and contact the metal component underneath and thereby generate PIM.
The present invention provides a novel blanket seaming tape having a construction for preventing RF energy from penetrating the seams between adjoining RF shielding thermal blankets.